It's like when Doom: Eternal and Animal Crossing: New Horizons came out at the same time. People find it funny when two disparate properties release at the same time in the same space. Especially in the movie theater where double features are not uncommon, there's a lot of people who want to see both and think the high contrast in tone and style of movies will make for an interesting experience.

Personally I anticipate Oppenheimer being a pretty bleak experience so going immediately to Barbie after will be a fun palate cleanser

Clearly barbie is a product of nuclear falloff.

That's the point. The release at the same time. The dark, challenging Oppenheimer movie about a serious topic and the Barbie movie. Barbie as a brand is perceived as simple and childish. And while the movie looks to be more, people got already a kick out of it when it was announced to run on the same day. So the joke is the contrast between these entirely opposite views on these two names.

Also Barbie being Barbie, she probably was a nuclear theoretical physicist at some point.